Meta·Software Engineer·Technical Phone Screen·Senior

Senior

May 2026

Summary

Meta Data Engineer technical screen, one SQL question that looked manageable until I actually had to write it out. The join logic wasn't bad but getting the percentage calculation right in a single query took more mental gymnastics than I expected.

Questions Asked (1)

Given a copies table (with copy_id and condition) and a checkouts table (with copy_id, checkout_date, return_date, and renewal_count), write a single SQL query returning two columns: the count of active checkouts for copies in 'good' condition, and the fraction of those active checkouts where renewal_count exceeds 2.

Data ModelingProduct Analytics & Metrics

Author's notes

I joined the two tables on copy_id, filtered for condition = 'good' and return_date IS NULL, then used COUNT(*) for the first column.

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AI HintsAI Generated

Suggested Approach

First, clarify the definition of 'active checkout' (e.g., return_date IS NULL) and confirm the condition value 'good'. Then, write a single query that joins the copies and checkouts tables, filters for good condition and active checkouts, and computes the count and the fraction using conditional aggregation (e.g., SUM(CASE WHEN renewal_count > 2 THEN 1 ELSE 0 END) / COUNT(*)).

Pro tip: Mention that you would validate the definition of 'active' with the interviewer, as it could mean return_date IS NULL or return_date > CURRENT_DATE, and that you'd handle potential division by zero by using NULLIF or a CASE expression.

1. Clarify requirements and assumptions

Confirm what 'active checkout' means (e.g., return_date IS NULL) and that 'good' condition is an exact match. Also, clarify if the fraction should be a decimal or percentage.

2. Identify necessary tables and join condition

Use the copies table to filter condition = 'good', and join to checkouts on copy_id. Ensure the join is correct (e.g., INNER JOIN).

3. Filter for active checkouts

Apply a WHERE clause to select only active checkouts, such as return_date IS NULL.

4. Compute the two metrics

Use COUNT(*) for the total active checkouts, and for the fraction, use conditional aggregation: SUM(CASE WHEN renewal_count > 2 THEN 1 ELSE 0 END) * 1.0 / COUNT(*). Handle division by zero with NULLIF.

5. Write and explain the final query

Present the complete SQL query, explaining each part. Optionally, discuss performance considerations like indexing on copy_id and condition.

Key Points to Mention

Definition of 'active checkout' (e.g., return_date IS NULL) and confirmation with interviewer
Use of conditional aggregation (CASE WHEN) to compute the fraction
Handling division by zero using NULLIF or CASE
Ensuring the fraction is computed as a decimal (multiply by 1.0 or cast to float)
Filtering copies by condition = 'good' before joining to avoid unnecessary rows
Potential need for indexing on copy_id and condition for performance

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.